1. Field of the Invention
The present invention relates to optical filters and, in particular, to optical add/drop filters.
2. Discussion of the Related Art
With existing fiber optic networks, there is often the need to increase information transmission capacity. However, both physical and economic constraints can limit the feasibility of increasing transmission capacity. For example, installing additional fiber optic cable to support additional signal channels can be cost prohibitive, and electronic system components may impose physical limitations on the speed of information that can be transmitted. One way to increase the capacity of an existing fiber optic link without modification to the fiber itself is by multiplexing multiple signals via wavelength division multiplexers (WDMs). The use of WDMs provides a simple and economical way to increase the transmission capacity of fiber optic communication systems by allowing multiple wavelengths to be transmitted and received over a single optical fiber through signal wavelength multiplexing and demultiplexing. The demultiplexed signals can then be routed to the final destinations.
Dense WDMs (DWDMs) can be utilized to further increase information transmission capacity. In a DWDM system, multiple optical signals, each having a different channel or wavelength, are multiplexed to form an optical signal comprised of the individual optical signals. The signal is transmitted over a single waveguide and demultiplexed at a receiving end such that each channel wavelength is individually routed to a designated receiver. Through the use of optical amplifiers, such as doped fiber amplifiers, optical channels can be directly amplified simultaneously, thereby facilitating the use of DWDM systems in long-distance optical systems. DWDMs can be made using techniques, such as disclosed in commonly-owned U.S. Pat. No. 5,809,190, which is incorporated by reference in its entirety.
Because DWDMs can multiplex and demultiplex large numbers of communication channels, e.g., 8, 16, or even 32 discrete communication channels onto a single optic fiber, and transmit these channels over long distances, some of the channels may be desired at intermediate nodes before demultiplexing. Selected channels from the multiplexed signal are extracted or "dropped" and routed to desired nodes for use, such as for transmission to users coupled to the node. However, other nodes along the single optic cable path or at the demultiplexing node may also want to utilize the extracted signals. In addition, intermediate nodes may generate signals for transmission along the single optic cable. Accordingly, extracted or newly generated signals are inserted or "added" into the multiplexed signal. This extracting and inserting of optical signals is generally referred to as add/drop multiplexing and is typically carried out with devices such as optical add/drop filters (OADFs) or OADF modules. FIG. 1 shows a generalized WDM system having an OADF module 100. Signals having wavelengths .lambda..sub.1, .lambda..sub.2, . . . , .lambda..sub.N, are multiplexed onto a single optical fiber 110. OADF module 100 drops a signal at the selected wavelength, e.g., .lambda..sub.2, for routing to desired destinations. OADF module 100 also adds back the signal at wavelength .lambda..sub.2 to the multiplexed signal for continued transmission. The multiplexed signal is then demultiplexed into individual signals at wavelengths .lambda..sub.1, .lambda..sub.2, . . . , .lambda..sub.N. Note that any number of OADF modules 100 can be inserted along the multiplexed signal.
One type of OADF module 100 is shown in FIG. 2, which utilizes two non-absorbing interference filters 210 and 220. Filters 210 and 220 comprise dielectric layers or coatings having refraction indices and thicknesses so that filters 210 and 220 transmit a certain portion of the spectrum of the incident radiation and reflect the remaining portion. For example, filter 210 receives a multiplexed optical signal having wavelengths .lambda..sub.1, .lambda..sub.2, .lambda..sub.3, and .lambda..sub.4, the dielectric coating transmits the signal to be extracted, e.g., at .lambda..sub.2, and reflects all other signals, e.g., at .lambda..sub.1, .lambda..sub.3, and .lambda..sub.4, to filter 220. Filter 220 also receives the signal to be inserted, e.g., at .lambda..sub.2, and combines the signal with the multiplexed signal.
FIG. 3 shows another type of OADF module utilizing two optical circulators 310 and 320 and fiber Bragg grating 330 coupled between circulators 310 and 320. The multiplexed optical signal, e.g., at wavelengths .lambda..sub.1, .lambda..sub.2, .lambda..sub.3, and .lambda..sub.4, enters optical circulator 310 at an input port 311 and is transmitted along an optical fiber 340 toward fiber grating 330 via an input/output port 312. Fiber grating 330 reflects the signal to be extracted, e.g., at .lambda..sub.2, back to optical circulator 310 via input/output port 312, where the signal is dropped at an output port 313. Meanwhile, the unreflected portion of the multiplexed signal, i.e., channels at .lambda..sub.1, .lambda..sub.3, and .lambda..sub.4, travel through fiber grating 330 and enter optical circulator 320 via input/output port 321. A signal to be inserted, such as a signal at .lambda..sub.2, is inserted to optical circulator 320 at input port 322. This signal is transmitted back along fiber 340 to fiber grating 330, where it is reflected back to circulator 320 and inserted into the multiplexed signal. Thus, a signal with all channel components at .lambda..sub.1, .lambda..sub.2, .lambda..sub.3, and .lambda..sub.4, is transmitted out of circulator 320 at an output port 323.
While the OADF modules discussed above are effective for dropping and adding a single optical channel, problems arise when more than one optical channel is dropped and added from the multiplexed signal. One way to add and drop multiple optical signals is to add additional OADF modules to the single OADF modules of FIGS. 2 and 3. For example, each of the additional OADF modules can have dielectric coatings that transmit signals at a distinct wavelength or fiber gratings that only reflect signals at a particular wavelength. Thus, another OADF module can be coupled to the output of the single OADF module of FIGS. 2 and 3, where the second OADF module drops the channel at .lambda..sub.1 at one filter or circulator and adds a channel at .lambda..sub.1 at another filter or circulator. Accordingly, by cascading N OADF modules, N channels can be dropped and added along the multiplexed signal. However, where N is large, such as with DWDM applications, the system can be large and costly, requiring large numbers of interference filters or optical circulators and fiber gratings. In addition, the dielectric coatings and fiber gratings do not completely reflect or pass signals. Therefore, the resulting multiplexed signal out of each OADF module experiences some signal loss, which compounds as the signal travels through each subsequent OADF module. As a result, systems adding and dropping many channels can experience substantial signal degradation.
FIG. 4 shows an OADF module capable of adding and dropping multiple channels, but having only two optical circulators 410 and 420. The OADF module is similar to that of FIG. 3, except that each optical circulator drops or adds multiple channels via an exit port 411 or input port 412, respectively, and that multiple fiber gratings are coupled along the optic fiber between the two circulators. For example, optical circulator 410 drops channels at .lambda..sub.1, .lambda..sub.2, .lambda..sub.3, and .lambda..sub.4 from an N-channel multiplexed signal, and optical circulator 420 adds channels at .lambda..sub.1, .lambda..sub.2, .lambda..sub.3, and .lambda..sub.4. Fiber Bragg gratings 431-434 are tuned to reflect signals at .lambda..sub.1, .lambda..sub.2, .lambda..sub.3, and .lambda..sub.4, respectively. Because fiber gratings 431-434 do not completely reflect signals at the tuned wavelengths, leakage components at the tuned wavelengths pass through the associated fiber gratings. These adverse effects increase as the number of channels to be added and dropped increases, which necessitates an increased number of tuned fiber Bragg gratings.
Accordingly, an optical add/drop filter is desired that are capable of adding and dropping multiple signals without the disadvantages of conventional OADF modules discussed above.